Embryonic and posthatched differentiation of bursal secretory dendritic cells, which
express vimentin intermediate filaments, were studied with anti-vimentin (clone 3B4)
and anti-cytokeratin (clone Lu5) monoclonal antibodies. Anti-cytokeratin staining
revealed that medullary reticular epithelial cells formed a continuous network at
every age, whereas the vimentin positive cells were single and showed dendritic appearance.
On the basis of location, number, shape, polarized appearance, and Ia staining, the
vimentin-positive cells and secretory dendritic cells appeared to be the same cell.
Secretory dendritic cell precursors entered the bursal epithelium between 11 and 13
days of embryogenesis. The first vimentin positive cell appeared in the bud of 14-day
embryos. Bud formation preceded the appearance of vimentin-positive cells. These observations
suggested that the secretory dendritic cell precursor did not express vimentin when
it entered the epithelium. Between 15 days of embryogenesis and 2 weeks of posthatch
development, the changes in vimentin staining pattern revealed a cytological differentiation
of the vimentin-positive cell. During rapid bursal growth, the number of secretory
dendritic cells (vimentin-positive cells) increased about 18 times possibly by proliferation
of vimentin-negative precursors in the epithelial arches of the corticomedullary border.
